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Implanted defibrillators save lives by shocking hearts back into a normal rhythm. But sometimes a defibrillator can go off when it's not necessary, delivering a shock that feels like a kick in the chest.

The study found that reprogramming defibrillators to be less sensitive to irregular heart rhythms reduced the number of inappropriate shocks, while also reducing mortality. The study was presented at a meeting of the American Heart Association Scientific Sessions and is being published in the New England Journal of Medicine.

"Inappropriate shocks can be painful and psychologically traumatic to patients," said cardiologist Dr. Peter Santucci, medical director of Loyola's Implant Device Program. "It's important to reduce these shocks, and results of this study will help us to do this, while also potentially improving patients' survival."

Santucci enrolled Loyola patients in the multi-center international trial. Dr. David Wilber, director of Loyola's Cardiovascular Institute, is a co-author of the paper. The trial is known as MADIT-RIT (Multicenter Automatic Defibrillator Implantation Trial -- Reduce Inappropriate Therapy.) First author is Dr. Arthur J. Moss of the University of Rochester Medical Center.

An implantable cardioverter defibrillator (ICD) is about the size of a pocket watch, and is implanted below the collarbone. Wire leads connect to the heart. The device is designed to protect against tachyarrhythimas -- quivering, superfast heartbeats that prevent the heart from pumping blood effectively.

When the heart goes into a tachyarrhythimia, the ICD's pacemaker is activated. If the pacemaker fails to restore a normal rhythm, the ICD then delivers a powerful electric shock that jolts the heart back into a normal rhythm. But previous research, cited in the new paper, found that ICDs are inappropriately activated in between 8 percent and 40 percent of patients.

Under conventional programming, an ICD may be activated if the heart beats 170 to 199 beats per minute for 2.5 seconds or at least 200 beats per minute for 1 second. If the heart rate does not slow within 5 to 10 seconds, a shock may be delivered.

The study included 1,500 patients, who were randomly assigned to three groups. The first group had ICDs with conventional programming. In the second group, the ICDs would not activate unless the heart beat at least 200 beats per minute. In the third group, the ICDs were programmed to have longer delays before activation (for example, 60 seconds delay when the heart beats 170 to 199 beats per minute).

After an average follow-up of 1.4 years, patients in the second group had a 79 percent reduction in first-time inappropriate ICD activation. Patients in the third group had a 76 percent reduction in first-time inappropriate activation.

There was a 55 percent reduction in deaths in the second group and a 44 percent reduction in deaths in the third group.

Wilber is a professor and Santucci is an associate professor in the Department of Medicine, Division of Cardiology of Loyola University Chicago Stritch School of Medicine.

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